Congressional investigators have pieced together a series of decisions that led State Department officials to inexplicably draw down security in Libya last year even as threats and attacks against Western diplomats were rising in the violent, chaotic city of Benghazi where America’s ambassador was killed last Sept. 11.

The investigators have determined that between May and September, the department reduced the number of Mobile Security Deployment teams from three to one, thinning the potential U.S. security officers available to protect diplomats by at least twelve, the Washington Guardian has learned.

In addition, the lone remaining six-member Mobile Security Team in Libya at the time Ambassador Chris Stevens was killed was detailed primarily to training Libyan security officials rather than providing force protection to U.S. officials, the sources told the Washington Guardian.

The two-thirds reduction in the number of special six-member security teams came in addition to decisions by the administration not to place a Marine security contingent at the main embassy in Tripoli and to send home in August a special military attachment that was also providing security for U.S. officials in Libya, the sources said, speaking only on condition of anonymity because they were not authorized to discuss the specifics of the department’s security planning.

The commander of the military team sent home in August has already told Congress he requested that his team be extended or replaced but his request was denied.

The dual drawdowns help explain why the U.S. compound in Benghazi and its visiting ambassador were protected on Sept. 11 only by a few U.S. security agents and some retired Navy SEALS who worked for the CIA, a force that fought valiantly but was significantly outmanned by the al-Qaida inspired terrorists who struck with guns, rocket-propelled grenades and mortars. Local Libya security guards also at the location proved incapable of repelling the attack, officials have said.

At hearings planned for Wednesday, lawmakers in both the House and Senate plan to confront Secretary of State Hillary Clinton about the security reductions, which stood in stark contrast to the decisions made by one of America’s closest allies.

British officials pulled their diplomats from Benghazi after a brazen summer attack against one of their own, and they had not yet returned them when the U.S. special mission in that city was attacked the night of Sept. 11, the 11th anniversary of the deadly 2001 terror attacks in Washington and New York City

In fact hours before he and three other Americans died in the attack, Stevens reported to Clinton in a diplomatic cable that he had conferred with British authorities, and they would not be making a decision on whether to return until at least October.

In that same cable, Stevens painted a portrait of Benghazi that was violent, chaotic and suffering through constant attacks from Islamist extremists that ranged from car bombings to explosive attacks on power lines.

Stevens also warned that the loyalties and capabilities of local Libyan militias helping to protect U.S. officials in the region were also increasingly dubious, specifically reporting that one group of militia leaders had threatened to pull their security if Americans continued to support a particular candidate for Libyan prime minister.

Stevens’ final, fateful cable was hardly the first sign of deteriorating security. Congressional investigators have assembled a list of more than a dozen threats or attempted attacks on Western diplomats in the Benghazi area in the months before the attack on the compound.

The prior episodes included a gunfire incident near the U.S mission in Benghazi in March, an explosives attacks against the compound in June that blew a hole in a security wall, a grenade attack on the International Red Cross station in Benghazi and an attack on the British ambassador’s motorcade in the city.

U.S. officials clearly understood the deteriorating conditions, congressional investigators found, because they increased the hazard pay for State Department officials serving in the region, the sources said. But at the same time, they drew down the security assets sent by both the State Department and the military.

Obama administration officials declined comment, referring a reporter to the State Department’s recent report on the episode. That report cast blame on the entire agency for keeping a woefully inadequate security posture for the threats faced in Libya.

While the primary focus at Wednesday’s hearings will be on the State Department bureaucracy, Democrats plan to blame House Republicans for failing to approve in the recent Sandy recovery bill more diplomatic security funding for the State Department.

“I know from personal experience that there are many thoughtful Republicans in positions of responsibility in the House who want to do the right thing,” Sen. Patrick Leahy, D-Vt., said Tuesday in testing the new line of attack. “Yet a small but apparently dominant obstructionist cadre within the Republican ranks seemingly is willing to push the bounds of irresponsibility to ever greater heights.”

Overall, Congress since 2007 had increased diplomatic security funding for the department by about 27 percent, officials have said.